Question

E.coli about to replicate was placed in a medium containing radioactive thymidine for five minutes. Then it was made to replicate in a normal medium. Which of the following observation will be correct?

• A. Each strand is half radioactive
• B. Both the strands of DNA will be radioactive
• C. One strand will be radioactive
• D. None of the above

Answer 1

The correct option is C One strand will be radioactive

DNA replication is semiconservative in which a DNA strand serves as a template for the synthesis of a new strand and produces two new DNA molecules, each with one new strand and one parental strand. Treatment of E. coli with tritiated thymidine incorporates tritium in place of a hydrogen atom in thymidine. DNA isolated just after treatment and before cell division shows radioactivity in both strands; option B is incorrect. DNA replication of this parental DNA with tritiated thymidine synthesizes daughter DNA strand. Since the daughter DNA strand carries normal thymidine, the new DNA duplex is hybrid of the parental strand with tritiated thymidine and new DNA strand with normal thymidine; option C is correct. The second round of cell division produces 50% normal DNA and 50% DNA intermediate DNA; option D is incorrect. DNA replication does not break the parental DNA strand which rules out option A.

Thus, the correct answer is option C.